Battery-supplied systems have become increasingly popular in recent years, thanks to the widespread diffusion of a number of portable devices (for example, mobile telephones). The batteries allow using such systems without the need to plug into an electrical socket. However, any battery has a limited duration (for example, some hundreds of hours), so that it must be recharged after a period of use.
Different kinds of battery-charger devices are known in the art. In one known architecture, the battery-charger implements an analog closed-loop, which is used to regulate the current that is supplied to the battery during the charging process. For this purpose, the charging current is converted into a voltage, which is measured by a corresponding sensor. The sensing voltage is compared with a reference voltage; a resulting error voltage is amplified and then used to regulate the charging current accordingly. Typically, a circuit implementing the above-described feedback loop (for controlling the battery-charger) is integrated in a chip of semiconductor material.
The sensing voltage to be measured has a very low value (on the order of some mV); moreover, the sensor suffers a significant direct current (DC) offset, which can swamp any useful sensing voltage. As a consequence, the sensor is commonly stabilized with a chopper-based technique to reduce its offset. The chopper-stabilized sensor operates in a track-and-hold (T/H) mode, under the control of a periodic clock signal. In particular, during a first half-period (tracking phase) the sensing voltage is transferred to an output of the sensor, and during a second half-period (holding phase) the sensing voltage is sampled and held. In the holding phase, the connections to a resistor used to convert the charging current into the sensing voltage are inverted, so that the opposite polarity offsets cancel.
A problem with such battery-chargers with a closed-loop architecture is in ensuring their stability. For this purpose, it is necessary to define a model simulating operation of the battery-charger. A stability analysis is then performed on that model; a compensation network (for example, consisting of a feedback capacitor) is designed by exploiting classical techniques well known in the art. An additional difficulty is due to the fact that the chopper-stabilized sensor cannot be simulated directly, but its transfer function must be linearized to represent the track-and-hold operation.
A drawback of the solution described above is that the compensation network cannot be updated once it is integrated into the control circuit of the battery-charger. Moreover, the stability of the battery-charger strongly depends on the structure of a regulator that is external to the control circuit; therefore, different control circuits must be used whenever the regulator is changed. Further, the compensation capacitor occupies a large area of the chip in which the control circuit is integrated.